1. Field of the Invention
The present invention relates to an apparatus that forms picture images by making a plurality of picture images overlap each other, for example, a color picture image formation apparatus by which picture images of two or more colors are made to overlap each other.
2. Description of the Related Art
Various kinds of printers that form multiple picture images by making two or more picture images overlap each other have been proposed up to the present time. As a typical example, an image formation apparatus which is capable of irradiating a plurality of photosensitive drums with corresponding light beams (laser beams) to form color picture images by a photoelectric process has been proposed. In this type of apparatus, each of the color picture images which are formed on each photosensitive drum and transferred to a common transfer belt. The images so transferred sometimes do not come into registration due to mechanical mounting errors between the photosensitive drums, errors in the length of a light path of each laser beam, variations in the light path, etc. For this reason, the prior art proposed the following: forming patterns for registration correction on the transfer belt and reading such patterns by a sensor composed of CCDs or the like from each photosensitive drum. Registration variations on the photosensitive drum corresponding to each color are detected. Based on the detection, electrical correction is made on image signals to be recorded, or optical system such as a mirror or the like disposed in a light path of a laser beam is moved and positioned by a drive means so as to correct variations in the length of a light path or in the light path.
However, in the above-mentioned conventional picture image formation apparatus, where a registration pattern is read out by the above sensor, the storage time of the sensor is short, particularly in a high-speed color copier, since the reading is performed in a time period equivalent to that of the usual copy process. Thus, the S/N ratio of signals read becomes poor. Even if the registration pattern is read, a special-purpose picture-image processing means is required in order to remove noise components after the reading and to ensure registration correction.
In a high-speed reading, another problem occurs if, for instance, the light quantity of an illumination lamp is increased in order to improve the S/N ratio of registration pattern signals, power consumption increases considerably in comparison to power consumed at normal times.
These problems occur not only for a photoelectric color printer but for other prints where two or more picture images are made to overlap as well, such as an ink jet type or a thermal transfer type printer.